We are pioneering an RNA-based switch technology called structurally interacting RNAs (sxRNA) which utilizes post-transcriptional gene regulation as a reporter for miRNA detection. Normally, RNA-binding proteins (RBP) associate with a 3' stem-loop structure to facilitate translation of an upstream coding region by as much as an order of magnitude. It is possible to modify the mRNA to modulate translation of the reporter gene by controlling the binding of RBP. This is accomplished by creating a trans-acting three way junction with an additional RNA, such as a miRNA, binds and stabilizes the functional structure by base-pairing with the flanking regions of the custom designed stem-loop. Our goal with this proposal is use this selective expression capability to create homogenous populations of cells in culture, focusing specifically on stem cells. We plan to create a procedure, sold as a kit, that would eliminate unwanted cells from a differentiated cell population. We will accomplish this by using miRNA expression in the desired cells to turn on translation of an antibiotic gene thereby maintaining the desired cells. Success with this product development effort will result in a product that has broad appeal to stem cell researchers both for the benefits of increased homogeneity of stem cell cultures and potentially faster development of stem cell related lines, and position us to further enhance the technology for use in CHO cells lines for cGMP cell culturing.